Traditionally, single walled tanks have been
to store and transport liquid materials. widely used Such tanks have usually been constructed from steel, because of the affordability and structural stability of single walled steel tanks. Single walled steel tanks have been used underground, above ground, on railroad cars, and on ships as holding tanks. They have been used to hold a wide variety of materials, including gasoline, toxic and hazardous wastes, and even benign substances such as tomato juice. Single walled steel tanks, however, are prone, over time, to deterioration and failure. If used underground, they may rust, remove space both from the outside and from the inside. Such deterioration is difficult to detect, because it is underground. If used above ground, deterioration is usually less rapid and may be more visible, if occurring on the outside of the tank. Nonetheless, pinhole size leaks are very difficult to detect, even on the outside of above ground tanks.
Single walled tanks made from materials other than steel are prone to similar and/or other problems, all involving undetected tank failure. Even a minute failure, undetectable via visual detection, can be extremely serious, if very toxic or hazardous materials are being stored in the failed tank. Just one gallon of gasoline can contaminate one million gallons of drinking water.
Yet gasoline is one of the safer materials commonly stored in existing single walled tanks as other materials, such as dioxins, fluorinated and chlorinated hydrocarbons, ammonia, toluene and so on, are far more dangerous.
Moreover, it is estimated that there are between 7 and 15 million underground tanks in the United States storing liquids that are regulated by the EPA. Further, there are millions of additional above ground storage tanks, railroad cars and ship transport tanks which are used to store or to transport liquids governed by EPA regulations. Environmental disasters have already occurred. Highly publicized leaks and spills of hazardous and toxic materials have clarified the need to regulate and prevent spills and other leakage of such materials into the environment.
In 1983, the EPA began regulating the storage of gasoline and certain other materials in underground tanks. Many states have passed stricter regulations extending to above ground storage of various materials. This trend to regulate the storage and transport of hazardous or toxic materials in tanks of all types is almost certain to continue.
The trend is to require storage and transport of hazardous and toxic materials in double walled tanks with a monitoring device installed to detect leakage between the walls of the tank, thus detecting the failure of either wall of the tank. The EPA and many states have already adopted such requirements for the storage or transport of many materials in particular situations. This trend will almost certainly result in more regulation, not less.
Thus, there is an urgent need to replace or convert literally millions of existing single walled storage tanks into double walled storage tanks with monitoring systems to detect leakage between the walls of the tank.
Nonetheless, the cost of replacing existing tanks is usually quite high. In the case of underground storage tanks, the old tank must be dug up and disposed of, and a new double walled tank must be purchased and installed. Replacing above ground storage tanks does not involve the extensive dig-up costs, but still requires the total capital cost of replacing the old tank with a more sophisticated and expensive tank. These capital intensive replacements would be even worse for railroad cars and large ship holding tanks.
Conventional methods exist for digging up existing underground storage tanks and retrofitting them with an added outer wall and monitoring system meeting all current EPA and state requirements. These same methods may be used to retrofit outer-coat above ground storage tanks, railroad cars, and large ship holding tanks.
Nonetheless, such methods suffer from several important drawbacks. First, digging up existing tanks and retrofitting them is an expensive proposition. It is almost as expensive, in many cases, as simply digging up the old tank and replacing it with a new tank. In such a case it usually makes sense to go with a brand new tank.
Second, in the case of above ground tanks, railroad cars, and ship holding tanks, the outer layer of the original tank is exposed, meaning that it must have certain minimum structural and damage resistance characteristics. If it has been decided that a new outer layer wall must be added to comply with EPA regulations, this generally results in the new outer layer being more costly than it would need to be if it were installed on the inside.
Further, not all retrofitted outer-wall double walled tanks have proven acceptable. Simple polyethylene liners have proven difficult to monitor for tank wall failure. Double walled fiberglass reinforced plastic tanks have also been used, but generally rejected because they generally do not have a sufficient gap between the walls to monitor and act on leaks between the walls before actual leakage to the environment has occurred.
Methods also exist for retrofit coating the inner surface of an existing single walled tank to create a double walled tank. Nonetheless, such existing methods have not resulted in acceptable tanks due to lack of durability of the inner surface lining used and/or due to permeability, albeit slight, of the lining, which can give false alerts to any leak sensing device being used.
Thus, an improved method for in situ conversion of old single walled tanks into acceptable double walled tanks by retrofit inner-coating existing single walled tanks is desperately needed. Ideally, such a method would: (1) be simple and cost efficient to implement; (2) not require costly, rare or dangerous materials; and (3) result in an acceptable double walled tank meeting all current and foreseeable regulatory requirements.
The most important attempts to solve these problems by previous methods are described in the following patents. USPN 4,817,817, issued to David T. Palazzo on Apr. 4, 1989, discloses a typical method of in-factory manufacturing of a double walled tank with interstitial monitoring for leakage. His method includes applying a release agent over the single walled tank and then applying a substantially rigid outer sheath of a curable synthetic resin thereover that, when cured, may be broken free from engagement with the inner tank.
German Pat. No. DT 2658-968, issued to Harry Haase on Jun. 6, 1978, discloses a double walled underground storage tank for liquids having walls constructed of glass-fibre-reinforced plastics, with an intermediate leakage control space filled with an air and liquid permeable material which also supports both walls against each other. The leakage control space is filled throughout with a plastic containing concrete which forms the static-load bearing layer of the tank. The plastic containing concrete forms an integral unit with the inner and outer walls.
European Patent Application 0069944, filed Jul. 3, 1982, discloses a method for restoring steel containers including coating their inner surfaces with a laminated resin, the strength of which corresponds to that of the container. A spacer layer is applied by spraying with a reinforcing particle-containing resin composition.
In summary, prior art and existing storage and transport tanks have been shown to be extremely prone to failure and environmental disasters of varying magnitudes. Accordingly, the EPA and many states have passed various regulations regarding the storage and transport of toxic and hazardous materials in tanks. One very desirable tank configuration is a double walled tank with interwall monitoring to detect failure of either wall of the tank. Environmental spills and leaks may thus be curtailed before they occur.
Moreover, it is desirable to retrofit existing tanks because of the cost of replacing most existing tanks. It is also desirable to retrofit the inside surface of existing tanks, in most cases, because of savings incurred by not having to dig up buried tanks. Further, it is generally cheaper and more efficient to coat the inner surface of existing above ground tanks because inner coatings do not have to be load bearing and may be constructed less rigidly, again saving in cost and making installation easier. Finally, it is desirable to have a durable inner coating that is completely impenetrable to water and to the material being stored in the tank.
Accordingly, the primary object of the present invention is to provide an improved method for in situ conversion of existing single walled tanks into resultant double walled tanks for storing toxic and hazardous materials that can be monitored for failure of either wall of the double walled tank.
It is also an object of the present invention to provide a safe and environmentally desirable method for workers and installers while performing the in situ conversion of existing single walled tanks in accordance with the instant invention.
It is also an object of the present invention to provide a double walled tank by the method of the present invention whose inner wall is substantially impenetrable to water and to the material being stored therein.
It is also an object of the present invention to provide an improved double walled tank for storing toxic and hazardous materials.